The present invention pertains to blowout preventers used for sealing off oil or gas wells during emergency situations so as to contain dangerous blowouts. Several different general types of blowout preventers are known in the art. One of these is the ram-type blowout preventer, which typically includes a housing having a throughway for the drill pipe. First and second ram assemblies are mounted in the housing on opposite sides of the throughway. These assemblies may be reciprocated laterally inwardly and outwardly, i.e. toward and away from each other, and specifically, are moved inwardly to close the blowout preventer when it becomes necessary to seal off the well.
Ram-type blowout preventers may, in turn, be further subdivided into various types. One type is commonly referred to as the "pipe ram" type. In this type of apparatus, the ram assemblies, when moved laterally inwardly to closed position, simply seal about the outer diameter of the drill pipe thereby sealing off the annulus between the drill pipe and the walls of the well bore. Another kind of ram-type blowout preventer is commonly referred to as "shear ram" type. In this type, the ram assemblies, when reciprocated inwardly, actually sever the drill pipe and subsequently seal against each other in such a way that both the drill pipe itself and the surrounding annulus are sealed off in the sense that they are blocked from communication with the space above the blowout preventer. Both pipe rams and shear rams are typically included in a complete blowout preventer stack for a well.
Exemplary prior pipe ram assemblies are disclosed in U.S. Pat. No. 3,434,729 to Shaffer et al and in the 1982-83 Catalog of the Shaffer Division, NL Industries, Inc., pages 6151-6156, 6162, 6180, and 6189. Exemplary shear ram type blowout preventers are disclosed in U.S. Pat. No. 3,736,982 to Vujasinovic and in the 1982-83 Catalog of the Shaffer Division, NL Industries, Inc., pages 6157, 6163 and 6172. A common characteristic of these pipe ram and shear ram blowout preventers is that each individual ram assembly comprises a carrier or holder of some type and a ram block connected to the carrier for limited relative lateral movement by a lost motion connection. Thus, these structures may be distinguished from other forms of blowout preventers in which the individual ram structures are more or less integral, as exemplified by U.S. Pat. Nos. 4,132,267, 4,313,496, 4,341,264, 4,132,266, and 4,347,898.
One advantage of the former kind of construction, i.e. that in which each ram assembly comprises a relatively movable carrier and ram block, is that a seal may be installed between the carrier and ram block, and such seal may be compressively actuated by relative movement of the carrier and ram block. More specifically, and as described more fully in the aforementioned patents and catalog, as the ram assemblies are moved inwardly, such movement of the ram blocks will eventually be arrested, whereupon the carriers may continue moving inwardly for a limited distance. This actuates the seals between the blocks and carriers.
If the blowout preventer is of the "pipe ram" type, which simply seals about the outer diameter of the drill pipe, it is fairly convenient to design the structure so that the thicknesses of the ram block and carrier, measured longitudinally with respect to the drill pipe, are generally equal. This is desirable because it maximizes the surface area over which the forces may be distributed.
However, in shear ram type blowout preventers, due to the need to provide for actual severing of the drill pipe, and also to the significantly different sealing requirements, the carriers have traditionally been designed to provide a shelf-like structure which underlies and supports its respective ram block. Thus, the maximum thickness of the carrier was, of necessity, substantially greater than that of the ram block. Since the ability of the apparatus to seal off well pressures of a given magnitude is ultimately a function of the longitudinal thickness of the ram block, this proved to be a limiting factor in such prior art devices. To put it another way, it was necessary to make the carriers of the shear ram assemblies thicker than might ideally be desired in order to allow for adequate thickness of the ram blocks.
Another problem with such prior shear ram assemblies revolved around the fact that the shearing blades were mounted on the carriers, rather than the ram blocks, in an effort to avoid possible premature actuation of the seals. However, with this arrangement, and again due to limitations on the desirable thicknesses of various parts, it was conventional to mount the blades to the carriers with vertically disposed pins which, if broken, would almost surely fall downwardly, possibly interfering with other apparatus in and about the well.